Spectacle lens coatings should ideally be able to do everything

We use our spectacles every day without thinking about not only what our spectacle lenses can do optically, but also how robust they have to be to withstand what we subject them to. And that's the way things should be. No optical equipment is continuously exposed to the same mechanical strains and environmental impacts as spectacle lenses. Developers at Carl Zeiss work on constantly improving the finishes and coatings available for spectacle lenses. BETTER VISION asked René Stanke, the Product Manager for DuraVision® Platinum what modern spectacle lens finishes are all about.

BETTER VISION: When you have spectacles adjusted by an ophthalmic optician, it is normally the case that relatively little attention is paid to the choice of lens finish at the end of the consultation process. We tend to rely on the optician's recommendation. However, the finish has a critical influence on the service life of spectacle lenses, their vision and cleaning properties. What should spectacle wearers actually consider when choosing the coating?

René Stanke, the Product Manager for DuraVision® Platinum

René Stanke: You're right. Spectacle lens finishes can make a difference. These days, most people buy plastic lenses, but right from the outset plastic is not as hard, durable or scratch resistant as a mineral spectacle lens. As a result, I would definitely recommend a hard coating to extend the life of the lenses, and make the surface less sensitive to mechanical strains which can lead to scratches and therefore to a deterioration in vision.

A high quality anti-reflective coating is not only important to improve the look, but also to minimise distracting reflections through the spectacle lens, for example when driving at night or reading in natural and artificial light.

In addition, a good spectacle lens should not attract dirt, dust and fluff easily and should be easy to clean. Working together, modern spectacle lens finishes can make the lens a robust everyday object. We can only advise all spectacle wearers to ask their optician about the precise capabilities of a spectacle lens coating.

BETTER VISION: Carl Zeiss has now launched a new spectacle mineral finish - the DuraVision® Platinum. You were involved in the development. What is special about DuraVision® Platinum? What's new?

René Stanke: Based on our market research, we have optimised the entire range of coatings for ZEISS spectacle lenses. We are mainly talking about the following four product properties:

Firstly, we did a lot of work on the hardness of the spectacle lens surface. We are very proud to come up with the first plastic spectacle lenses with anti-reflective coating to have the same or even better hardness than comparable mineral spectacle lenses. Our measurements have shown that with DuraVision® Platinum we are producing three times harder spectacle lenses than in the past.

Secondly, we have significantly improved the anti-reflective coating performance. Despite the anti-reflective coating, every spectacle lens has what is known as a residual reflection, i.e. light that is reflected back by the spectacle lens and therefore does not find its way through the lens into the eye. Thus, the quality of anti-reflective coatings differs in terms of how low the residual reflection can be kept. Compared to previous lenses, we have succeeded in reducing this residual reflection by 20%.

Of course, we also wanted to maintain the outstanding, patented anti-static performance of ZEISS spectacle lenses, so that they do not get dirty quickly and attract significantly less dust and fluff. This is product feature no. 3.

The fourth is the exceptionally easy to clean spectacle lens surface provided by our water and oil repellant top coating. You can clean a spectacle lens with DuraVision® Platinum coating using our ZEISS cleaning products or simply under running lukewarm water, shake off the remaining water droplets and you will hardly need to wipe them with a microfiber cloth.

BETTER VISION: How can these improvements be tested - particularly the improved hardness properties?

René Stanke: We carry out intensive laboratory tests. For example, a diamond point is pressed into a conventional spectacle lens coating and the new comparison product. We measure the required force before the coating can no longer withstand the force and breaks. This enables us to determine and compare the hardness of individual coatings. DuraVision® Platinum achieved up to 50 per cent better results in this test than previous spectacle mineral finishes from ZEISS.

However, practical tests were more important to us, as a spectacle lens has to survive in day-to-day use, not in a laboratory. We assume a service life of two to three years, which we simulated in this test to demonstrate the wear resistance of our new coating. We got hold of "standard dirt" (which is also used in the vacuum cleaner industry for example), covered a spectacle cleaning cloth with it and "cleaned" spectacle lenses with a conventional coating and the new DuraVision® Platinum coating 600 times each with a defined pressure. The results were then analysed in detail under the microscope. The result: The spectacle lens with DuraVision® Platinum coating showed almost no damage, while the comparison product was very clearly scratched in this test.

BETTER VISION: Impressive. But this isn't something you should normally do to your spectacle lenses, is it?

René Stanke: No, please don't copy our tests. Despite everything that modern spectacle lens finishes can do, you should handle your spectacles with care. Over time, a large number of tiny scratches can make a spectacle lens appear opaque when you look through it, which impairs your vision and also significantly reduces the anti-reflective coating. Do not clean your spectacle lenses with clothing, tissues or similar items. We recommend running, lukewarm water or ZEISS cleaning spray and a microfibre spectacle cleaning cloth for day-to-day spectacle cleaning. From time to time, you can use moistened spectacle cleaning cloths from ZEISS, which do not attack the coating. Another tip: do not leave your spectacles in your car. The heat that can build up damages your spectacles. Don't wear your spectacles in the sauna and don't rest them on the lenses. This will increase the service life of your spectacles.

BETTER VISION: Absolutely, it would be a real shame if individually adapted progressive lenses did not last as long as they should.

René Stanke: Exactly. Unfortunately damage such as scratches cannot be polished out of plastic lenses and the lens cannot be given a new finish. This means that the spectacle lens is permanently damaged and a totally new one has to be made to restore full performance.

BETTER VISION: What personally impressed you the most about the development of the DuraVision® Platinum by ZEISS?

René Stanke: My highlight? There are two actually. Firstly, the fact that we created a plastic spectacle lens with the same hardness as a mineral lens or better, and secondly the fact that we managed to significantly improve the anti-reflective performance.

Here's how ZEISS precision spectacle lenses are given the new DuraVision® Platinum finish:

Firstly, the part-finished plastic product is worked on so that it has the optical and wear properties required to give the spectacle wearer the best visual experience. This is succeeded by turning and milling. The plastic lens is then polished to give optimum transparency.

The hard lacquer is then applied in an immersion bath, and is completely cured in a type of oven. This gives the lens a certain basic hardness and the lacquer provides an optimum link between the base lens and the anti-reflective coating.

The anti-reflective process with the individual anti-reflective coatings then starts in a high vacuum coating system. In total, eight ultra-thin layers are vapour coated. Metal oxides and semi-metal oxides are alternately deposited on the hard coated spectacle mineral surface.

During this process, high-energy ions, i.e. charged particles, are "fired" at the spectacle lenses based on a special method. These ions give off their energy when they strike the surface. This compresses the individual coating layers. This increases the hardness of the overall coating. Combined with the hard lacquer, this tailored coating provides the necessary robustness for the spectacle mineral coating.

In addition, the anti-static layer patented by ZEISS is applied, and finally the clean coat layer, which makes the spectacle lens water and oil repellent and easy to clean.

After dip coating with the hard lacquer, the vapour coating of the individual layers takes around three quarters of an hour in the coating system. But this is only the finish on the first side of the spectacle lens. The lens is turned and the second side is given the same treatment. The entire coating process must be carried out under dust-free clean room conditions. If even the slightest contamination occurs (e.g. a grain of dust) on the spectacle lens, it is rejected.

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